Hey palaemon! In this user forum, we don't have account access, so I haven't seen a bill with the discount applied (although honestly the Hookup codes started just over a month ago, so I'm not even sure that every Care or T-Force representative has had a chance to review a bill with a Hookup customer!). Typically, monthly discounts that apply an overall percentage off appear on the statement in itemized detail below the charges at the account level and line level, so as you review your statement you'll just be looking at the bottom of each section for that 20% off. Glad you were able to sign up with a sales rep who had a code to share!
The discount will show as an adjustment labeled Insider Hook Up. On an individual T-Mobile ONE plan it discounts $15 exactly.